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Info on Weber 40 DCOE sidedrafts

Discussion in 'The Hokey Ass Message Board' started by denis4x4, Oct 13, 2010.

  1. denis4x4
    Joined: Apr 23, 2005
    Posts: 3,766

    denis4x4
    ALLIANCE MEMBER
    from Colorado

    I scored a pair of Weber 40 DCOE side draft carbs for my latest project, a 1.8 SOHC for a huckster. I'm thinking that this maybe way to much carburetor for the engine. I have an Offy DP and Weber 32 two barrel on the shelf. The engine is .040 over, Crower Cam, ported, balanced and a lightened flywheel. Rear gears are 3:73 with a ZF 4 speed. Total weight will be around 2500 pounds.

    Too much carb?
     
  2. I used one 40 side draft Weber on my Fronty T speedster. The engine had an A crank which made it 200 cu in. The 40 was perfect, like it was made for the engine. I think, unless you have a lot of cam and compression, two would be too much.
     
  3. x2 on just one 40dcoe.. they are great carbs.. best fuel efficiency one can get. sell the other one for money twards the intake.
     
  4. Not too much carb by any means. I have run DUAL 40 DCOE's on Ford pushrod Kent 1.6 motors many times. 1600 Alfa motors, Fiats etc - many of them ran dual 40's, not to mention Datsun SOHC motors. Heck even DUAL 45's on hotter motors!
     

  5. The beauty of Webers is that they can be easily jetted and choked down to suit your needs. I've seen many twins on 4 cylinder cars.
    Make sure that your linkage in between them is solid...otherwise they will be out of sync and give you headaches when tuning.....often overlooked.
     
    Atwater Mike likes this.
  6. metalshapes
    Joined: Nov 18, 2002
    Posts: 10,716

    metalshapes
    Tech Editor

    Nope...

    40's are perfect, or can be...

    The 40mm size is only part of the story.

    Its the size of the butterfly's.

    The venturi's are removable, and that is what you use to size the carbs to the engine.

    The easy to get sizes for the venturi's are 28 to ( I think ) 32.

    I'd go for 28's or 30's, which might be a bit on the small side for your engine, but that only makes a little difference at top RPM/WOT.

    Having the venturi's a bit smaller gives a much better signal to the carbs, which makes them more forgiving and easyer to tune.

    And you'll pick up massive amounts of torque ( relatively speaking, its still a pretty small engine ) at low to mid RPM
     
  7. metalshapes
    Joined: Nov 18, 2002
    Posts: 10,716

    metalshapes
    Tech Editor

    Also, do not solid mount these carbs.

    You can mount them to rubber adapters, the way Alfa Romeo did it.
    But that means you'd have to modify your intake to get the flanges indexed right.( most aftermarket intakes have the same flange as the carbs )

    An easyer way is to get a Magard Kit out of England which is a piece that gets sandwitched between the carb throats and the intake runners and they have two gooves in them for O-rings.
    The carbs still mount on the same studs ( just longer ones ), with special washers that have a extra coil to form a stiff litttle spring.

    The Alfa rubbers need extra support under the carbs, the Magard kits dont.

    And get a good quality linkage or build your own.
    So the cabs wont go out of sync right away, when you start driving it...
     
  8. Soviet
    Joined: Sep 4, 2005
    Posts: 729

    Soviet
    Member

    Metalshapes,

    Why the soft mount? I ask because I've seen many mounted up to aluminum manifolds.
     
  9. metalshapes
    Joined: Nov 18, 2002
    Posts: 10,716

    metalshapes
    Tech Editor

    The engine vibration messes up the mixture.
     
  10. metalshapes
    Joined: Nov 18, 2002
    Posts: 10,716

    metalshapes
    Tech Editor

  11. I had a single 40 on my MGB 1.8 way back and it was too much carb for it even with the venturis changed out for smaller, I eventually went back to the SUs and have been very happy. I do think the DCOE is a great carb but for my set up the SU was just the easy way to go.
     
  12. CutawayAl
    Joined: Aug 3, 2009
    Posts: 2,144

    CutawayAl
    Member
    from MI

    I have seen more Webers that don't work as well as they should, than ones that are dialed in. There are good books available on tuning them. Be patient and do each step in order. Calibrated correctly, under most conditions you would think the car has EFI.

    As Metalshapes pointed out, on some inline 4 cylinder engines inherent vibration may require a flexible mount to avoid problems.
     
  13. metalshapes
    Joined: Nov 18, 2002
    Posts: 10,716

    metalshapes
    Tech Editor

    If you had one DCOE on a 4 cylinder, you didn't have one throat per cyinder, with an individual runner, like its designed to work.


    So the sizing is completely different.


    With a setup like what you had it reads more of the CFM, on a twin carb setup with individual runners it reads more of the pulses...
     
  14. rooman
    Joined: Sep 20, 2006
    Posts: 4,052

    rooman
    Member

    I am with you on this one Metalshapes. We ran as big as 45 DCOE carbs on 4 cylinder stuff in Oz. Choking and jetting is the key. Lynx in Australia also made an isolator package that allowed the carbs to "float" on the manifold.
    Back in the early 90's I was running an Offy engine with rigid mounted Webers on a dyno and at the point where the vibrations from the big crank get strong the brake specific numbers went into the toilet and fuel was actually running out of the velocity stacks as the floats fluttered up and down.
    The other advantage of running the dual carbs is that with the right manifold you end up with an IR package.

    Roo
     
  15. Thanks Metalshapes, If only I'd known this 20 years ago. I had a Datsun 1200 with a single 45 DCOE, with the smallest venturis available, a .444 lift cam and header with 2" exhaust. Very good carb, but inconsistent mixture problems at low RPM, especially after building solid engine mounts.:rolleyes: Car was fun and took a beating, used as a daily commuter (60 mile round trip) for about 2 years. Saw over 7000 RPM everyday.:D
     
  16. CutawayAl
    Joined: Aug 3, 2009
    Posts: 2,144

    CutawayAl
    Member
    from MI

    The MGB has siamesed potrs between #1 /#2 and #3 / #4. Even though the ports are combined, because of the engine's firing order, each bore of the carb "sees" each cylinder independently.
     
  17. zman
    Joined: Apr 2, 2001
    Posts: 16,594

    zman
    Member
    from Garner, NC

    Lot's of BMW 1600's and 2002's with twin 40's on basically stock motors and still getting close to 25mpg. They work right when setup right. One throat to one cylinder.
     
    Atwater Mike likes this.
  18. rooman
    Joined: Sep 20, 2006
    Posts: 4,052

    rooman
    Member

    Great minds think alike<G> I guess that we were typing at the same time.

    Roo
     
  19. draggin'GTO
    Joined: Jul 7, 2003
    Posts: 1,773

    draggin'GTO
    Member

    I ran a pair of 48IDA Webers with 37mm venturies on an 1835cc VW street engine, that's 112 cubes with two 583 cfm carbs on it. Had very smooth low-end and made good power from 1800 RPM all the way to 7000.

    How can that be, over 1000 cfm on a VW 4-banger? Individual runner manifolds, one carb barrel feeding one cylinder, the rules are completely different with IR manifolding.

    The AC Cobra had four of 'em on a 289 and those 48mm carbs could barely handle the airflow that the little 289 required. Not over-carbed even running the largest venturies available (45mm) for the 48IDA. For those unfamiliar with Webers, these very tuneable carbs used removeable venturies available in sizes from 32mm to 45mm. Over 2000 cfm on a 289, it works.

    If you ran these same 4 carbs dumping into a common plenum (like on a tunnel ram) instead of letting one carb throat only feed one cylinder (individual runner) then you'd be waaay overcarbed. The Webers are designed to be used on IR intake setups, not on a dual-plane or single-plane type intake.

    Not only do these IR intake setups have incredible low-end response and of course great top-end, they also get great gas mileage. As long as you're not tromping on the throttle and using a full shot from all eight accelerator pumps on a V8 (or four on a four-banger) all the time you'll pull some very good mileage numbers.

    Twin 40MM Weber 2-barrels on an 1800cc 4-cylinder engine couldn't be more perfect for the street, not even close to over-carbureting it. My very first hot street engine was a 1756cc (90mm X 69mm bore and stroke) VW flat-4 with twin 40DCNF 2-barrels running 32mm venturies.
     
  20. metalshapes
    Joined: Nov 18, 2002
    Posts: 10,716

    metalshapes
    Tech Editor

    No it doesnt.

    Not if the ports are siamesed like you say they are.



    Twin Webers on Siamesed intake ports like that are ( were ) very popular, OG Mini Coopers, etc,
    but the sizing is completely different from a one throat per runner/cylinder.

    So you cant say twin 40's on a 1800 would be too big, because one twin 40 didn't work on a head with siamesed ports...
     
  21. andyleonard
    Joined: Jun 4, 2010
    Posts: 22

    andyleonard
    Member
    from topanga ca


    Here's your good intel above. A BMW 2002 - for example - is 121 inches @ 140HP with 2 40s. If you want more power on 121 inches, you go up to 2 x 45. You don't need the rubber mounts...that's something the Italians came up with in the '60s...get the smaller chokes 28-30 and you'll have a ball. Buy some small main gas jets and some small idle jets and a good small drill set and have at it. Make big size changes until you see what it wants and what it doesn't want. Put a drop of solder in the jet and redrill if you get too big. When you get it right, order a final set of jets. The books have you changing emulsion tubes and air corrector jets and running around like a crazy chicken with boxes of jets and tubes and it's all too much work for a street car. Just get the main and idle gas jets close and you're good. And once you get them synced and jetted, DON'T TOUCH THEM.
     
  22. gimpyshotrods
    Joined: May 20, 2009
    Posts: 18,447

    gimpyshotrods
    ALLIANCE MEMBER

    I ran two Dellorto 45's (similar to the Weber) on my hot 1835cc VW motor back in the 80's. Worked great, and got great mileage (when/if I let up on the throttle).
     
  23. 283
    Joined: Aug 29, 2004
    Posts: 65

    283
    Member

  24. CutawayAl
    Joined: Aug 3, 2009
    Posts: 2,144

    CutawayAl
    Member
    from MI

    Look at which cylinders fire when on an inline 4 cylinder engine. The timing of the two cylinders sharing each siamesed port are separated by 360 degrees of crankshaft rotation. There is no overlap between the two. What the carb sees is a distinct and separate intake event from each cylinder sharing the port. That's why the same size carb throat works for an individual cylinder, or the two combined.

    The carb is sized for peak flow. When there is a carb throat for each cylinder, the throat must be big enough to supply the peak flow of each cylinder. When the ports are siamesed on an inline 4 cylinder there is no overlap between intake events. So, to satisfy peak flow the same size carb throat is needed for each siamesed port, but now only two throats are needed. When all four cylinders are combined into a common manifold, the intake events are still separate and individual with no significant overlap, so the carb throat size needed to supply peak flow for the entire engine(ignoring effects of the manifold's plenum volume) is still the same as would be neded for a single port, except now only one is needed. A V-8 with individual throats for each cylinder might have 350 CFM peak flow per cylinder. So, each cylinder has carb throat that flows 350 CFM. All eight carb throats combined would flow 2,800 CFM. The reason the engine isn't over carbureted is that each cylinder only has access to one 350 CFM carb throat.

    Consider V-8 engines with dual plane manifolds. The intake manifold divides the intake into two separate halves, with four individual events, just like a 4 cylinder engine. And, half of the carb feeds half of the cylinders. With the intake dividing the engine into two 4 cylinder engines, he carb throat size needed to supply one cylinder is adequate used to supply four cylinders. So, if one cylinder needed a carb that flows 350CFM to satisfy peak flow, 350 CFM will satisfy the flow requirements of the four individual intake strokes connected together in half of the intake manifold. Because there are two halves in the manifold, a 700CFM carb is needed to satisfy the flow requirements of each half of the manifold.

    This gets more complicated when there are overlapping intake events, as happens on a V-8 with siamesed ports, a single plane manifold, 360 manifold, cross ram, log maniofold, etc. With manifolds of that type the carb needs to be sized larger to compensate for the higher peak flow requirement resulting from overlapping intake events.
     
  25. Jonny69
    Joined: Jul 24, 2007
    Posts: 275

    Jonny69
    Member
    from England

    I have run twin 40's and a single 40 on various 1600's and 1700's over the last 10 years. I love the things. They are so simple and easy to get right if you know how.

    Get yourself this book: How to Build and Power Tune Weber and Dellorto DCOE and DHLA Carburettors by Des Hammill.

    Two DCOEs will not be too much. You can choke them down to suit the engine then jet it appropriately, as said. People who say they are too big have them jetted wrongly or their engine is too tired.

    I keep a little program on my website that calculates the jet and choke sizes for your engine. I have found it reliable but sadly I can't credit anyone for it because I can't remember where it came from:

    http://www.jonny69.co.uk/uploads/Carbcalc/carbcalc.exe

    You'll find it useful :)
     
  26. Kenneth S
    Joined: Dec 15, 2007
    Posts: 1,527

    Kenneth S
    Member

    I ran dual 45 DCOE Webers on a 2.0L S.O.H.C. (2000cc) in a 72 Pinto that I drove on the street, and also used to autocross with it on Hoosier road race slicks. I had 32mm venturies in them (I went with 45's rather than 40's so I could another engine with more hp). The 45's worked great.
     
  27. For two years my daily driver was a Datsun 620 with an L18 motor and dual 45 dcoe's. I had no problems with drive ability, warm up or anything else. 30,000 miles of going to school, work, truck in events. It was featured in Peterson's Pick Up and Vans magazine and took me everywhere, it was my only car.
    As for the rubber isolators, I never used or needed them on Datsun SOHC motors, but I did run them on both my Ford Kent motors and the BDA in my Escort MkI. Also did not use them on my BMW 2002
     
  28. metalshapes
    Joined: Nov 18, 2002
    Posts: 10,716

    metalshapes
    Tech Editor


    No, sorry...

    That is not correct.


    The MGB has a firingorder of 1342, and the intake ports are siamesed by combining #1 & #2, and #3 & #4.

    So 3&4 follow eachother, which will affect the flow and mixture of one.

    And 2&1 do too. ( 13421342, and so on...)

    That is why the intake manifolds you can buy for one DCOE on a 4inline with non-siamesed ports are usualy devided by 1&4 going into one throat, and 2&3 into the other...
     
  29. CutawayAl
    Joined: Aug 3, 2009
    Posts: 2,144

    CutawayAl
    Member
    from MI

    You are right about the firing order, but with 180 degrees of separation the principal I detailed still applies. Twice you have indicated I am mistaken. You either don't believe, don't understand, or don't want to understand, but I assure you there are well know principals i didn't dream up myself. Since both of us are sure of their facts, I'm willing to let anyone interested read what we have posted, then do the research to sort it out for themselves.
     
  30. metalshapes
    Joined: Nov 18, 2002
    Posts: 10,716

    metalshapes
    Tech Editor


    Let me correct you one more time.


    I do not believe you understand....
     

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